LU86962A1 - METHOD AND DEVICE FOR GIVING A CURVE TO A SHEET OF THERMOPLASTIC MATERIAL AND MANUFACTURE OF SHEET PANELS COMPRISING SUCH A SHEET - Google Patents

Description

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The present invention relates to a method for giving a predetermined curvature to a sheet of thermoplastic material in which such a thermoplastic sheet is curved by placing it against a face of a bending mold having a surface of the desired curvature, by heating the sheet thermoplastic and making it or letting it take the form of said surface. The invention also relates to a device for giving a predetermined curvature to a sheet of thermoplastic material.

10 Glass-plastic laminated panels are often used when the panel is likely to be subjected to impact. Thermoplastic sheets located inside such a panel can absorb energy upon impact against the panel, thereby effectively protecting the space behind such a panel used as a window. A thermoplastic sheet constituting an outer sheet of the panel will serve to prevent fragments of glass from detaching from the panel if it breaks, thereby reducing the risk of injury from broken glass. Curved panels of this type can be used as windshields or other glazing for vehicles of different types, including aircraft and automobiles.

It is obviously well known to use a thin flexible film of thermoplastic adhesive material, such as polyvinyl butyral, to form laminated glazing, but there is an increased tendency to also use more rigid thermoplastic materials since these have impact resistance properties appreciated! 30 wheats. Such signs are sometimes referred to as | "anti-aggression glazing" or "bullet-proof glazing", and i they can offer protection against the firing of weapons | fire.

! ! Polycarbonates and polymethacrylates are examples of relatively rigid thermoplastics. The fact that these materials are much stiffer than the materials that are usually joined to glass by

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<· Laminating, such as polyvinyl butyral, reveals a considerable problem: if the thermoplastic material does not conform well to the shape of a curved glass sheet to which it must be secured, reactive forces will appear in the thermoplastic material of the laminated panel and will tend to move it away from the glass and delaminate the panel. The problem is particularly acute when the laminated panel has a small radius of curvature and also around the edges of the laminated panel, where the stresses are concentrated. It is doubly unfortunate in this respect that vehicle windshields often have a greater curvature on or near their edges.

It is therefore desirable to give the sheets of thermoplastic material a predetermined curvature 15 before lamination. The thermoplastic can be heated and allowed to flex to conform to a bending mold of appropriate size and shape. But this does not solve the problem of delamination, especially around the panel, because it has been observed that, when the sheet of thermoplastic material cools after bending, its edges tend to curl up, so that it does not become not conform to the desired curvature at this location. Such a lack of conformity also increases the difficulties of forming a good bond between the sheets in a lamination process. Such a curl-up may naturally disappear during the lamination, but residual stresses remain in the thermoplastic material tending to cause delamination around the edges of the laminated panel. Even if the thermoplastic sheet is to be used as such without lamination, such curling of the edges is undesirable because it causes optical distortions at the edges of the sheet, and it makes laying in a frame more difficult.

One of the main objects of the present invention is to make it possible to give a predetermined curvature to a sheet of thermoplastic material which reduces, and in 3.

preferred embodiments, which eliminates this problem.

The present invention relates to a method for giving a predetermined curvature to a sheet of thermoplastic material in which such a thermoplastic sheet is curved by placing it against a face of a curving mold having a surface of the desired curvature, by heating the thermoplastic sheet and making it or letting it take the form of said surface, characterized in that such a thermoplastic sheet is oversized and is curved by placing it against a bending mold whose surface portion has the dimensions and the curvature reguises and is surrounded by a border, the thermoplastic sheet is heated and made or allowed to take the form of said surface portion while its oversized margins are supported by said border, and cooled then the thermoplastic sheet at a sufficiently slow pace to maintain such a shape, and in that the thermoplastic sheet is then brought to size ion.

By operating in this manner, the tendency of the thermoplastic to curl around its edges is reduced, and it is much simpler to achieve the desired curvature over the entire sheet which is to be used as or in a glazing. Such a curl can be limited to a relatively narrow peripheral band which is removed during the 4 dimensional setting, and can even be entirely eliminated. Consequently, a very high degree of conformity of the thermoplastic sheet is obtained with the predetermined curvature required, which ensures that the thermoplastic sheet is of the desired shape to facilitate frame mounting or subsequent lamination, and greatly reduces the tendency to delamination.

Another problem can arise when bending thermoplastic sheets. It has been noted that some surface ripple may appear in the thermoplastic after cooling, unless the thermoplastic has been able to cool at a very low rate. This implies a very slow production program which is 4.

incompatible with manufacturing on an industrial scale. Such a corrugated profile also increases the difficulties of forming a good bond between the sheets in a subsequent lamination process. Such waves can disappear naturally by pressing during the laminating process, but residual stresses remain in the thermoplastic material, which tends to cause delamination, especially of the edges. Even if the thermoplastic sheet is to be used as such, without lamination, such waviness is detrimental because it causes optical distortions.

Factors which seem to make this problem more acute are the relatively low thermal conductivity and the relatively high coefficient of linear thermal expansion of the thermoplastics particularly concerned.

The problem of obtaining or maintaining a high surface quality of the thermoplastic material while reducing the time required for cooling could therefore be solved by controlling the cooling rate only so that any irregularity in the contraction of the sheet thermoplastic during its cooling after bending can be compensated by a plastic flow inside the material. In other words, faster cooling can be tolerated while the thermoplastic is hotter and softer, but when the material cools and becomes more viscous, the cooling rate must be reduced.

Certain especially preferred embodiments of the invention are based on our discovery that this is entirely inaccurate. We have in fact discovered the exact opposite. For this reason, in the preferred embodiments of the invention, while the thermoplastic material remains in contact with said portion of the surface of the bending mold, it is made or allowed to cool from its maximum temperature during " 5.

\ a first period at a first cooling rate, and then for a second period at a second cooling rate which is higher than the first.

5 To our surprise, we have found that the cooling rate is more critical when the thermoplastic is near the maximum temperature at which it is heated for bending and that it is at such temperatures that the cooling rate should 10 be most carefully controlled, while higher cooling rates can be accepted at lower temperatures in order to obtain an acceptable total cooling time and to obtain a thermoplastic sheet of acceptable surface regularity. As has been pointed out, this is the opposite of what one might expect, and we cannot now explain it. the reason.

The advantages of precisely shaping a thermoplastic sheet at a predetermined curvature are particularly evident in cases where the sheet must then be joined to one or more other sheets having the same curvature to form a laminated panel, and therefore, in its second aspect, the invention relates to a method of manufacturing a laminated panel 25 transmitting light comprising at least one curved sheet of thermoplastic material secured to at least one curved glass sheet, this process comprising bending said sheets to a predetermined curvature and their subsequent joining together, characterized in that such a thermoplastic sheet is curved by a process as defined above, and in that the thermoplastic sheet and at least one curved sheet of glass are assembled and the assembly is subjected to temperature and pressure conditions to effect the solidarization sheets in a laminated panel.

Advantageously, the or each said thermoplastic sheet is dimensioned after joining in a \ 6.

y laminated panel. This facilitates precise cutting of the sheet of thermoplastic material to the correct size and shape.

One such thermoplastic sheet can be secured between two sheets of glass of the same shape. In this case, any residual stresses in the thermoplastic material will be distributed over one and the other of its joined faces. The concentration of stresses at any point of the glass / plastic junction will therefore be approximately half of what it would be if the thermoplastic material were only attached to one face of a glass sheet. The invention is therefore particularly advantageous in methods of manufacturing a laminated panel of the type in which thermo-plastic material is exposed on at least one main face of the panel.

Preferably, such a glass sheet and such a thermoplastic sheet are placed on either side of a thermoplastic adhesive film and a curved glass molding plate is brought into contact with the or each exposed face of thermoplastic material. , and the assembly thus formed is subjected to temperature and pressure conditions such that the sheets of glass and of thermoplastic material join together. The use of such a molding plate promotes a high surface regularity of the thermoplastic material in contact with it.

In such embodiments, it is preferred that the or each curved glass molding plate and the or each curved glass sheet intended to be part of a laminated panel are assembled and curved at the same time to the desired curvature. This is a very suitable way of ensuring that both or all of the glass sheets are given the same curvature.

The thermoplastic material may have been sheeted by extrusion. In general, extruded sheets have a fairly poor surface quality and they are not Λ 7.

flat enough to allow vision without distortion, this mistletoe excludes them from different uses. Any irregularity in the thickness of the thermoplastic material can make such a laminated panel unsatisfactory from a commercial point of view and even dangerous when good visibility is important, such as for vehicle windshields. For this reason, it is known to rectify any surface irregularity of the thermoplastic sheets by subjecting the sheets to a surface conditioning treatment.

It has been found that, although many surface irregularities can be rectified by heating the thermoplastic sheet so as to soften it while it is under pressure between a pair of molding plates, some surface ripple appears nevertheless in the thermoplastic material after cooling and return to atmospheric pressure at the end of the conditioning treatment, even if the thermoplastic material is allowed to cool at a very slow speed and if the thermoplastic material does not adhere to the mold plates with 20 which she is in contact.

Here again, this problem is all the more acute as the difference between the coefficients of linear thermal expansion of the molding plate and of the thermoplastic material increases, and it must also be attributable to this difference.

In order to solve this problem, it is preferred to condition the surfaces of a thermoplastic sheet by a method comprising sandwiching the thermoplastic sheet between a pair of glass molding plates and applying the temperature and temperature conditions to the sandwich. pressure so as to soften the thermoplastic material and to perform the required conditioning of the surfaces, and, before removing said molding plates, cool or allow the thermoplastic material to cool from its maximum temperature for a first period at a first speed cooling, and then for a second period at a second cooling rate which is \ 8.

higher than the first.

This also allows a reduction in the cooling time required, and at the same time allows a high degree of regularity of the thermoplastic sheet conditioned on the surface.

We highlight the use of glass molding plates because it is particularly advantageous due to the convenience with which such a molding plate can be shaped while having a high surface regularity, even if the The difference between the coefficients of thermal expansion of glass and many of the thermoplastics involved is quite large.

It is especially preferred that a sheet of float glass constitutes the molding surface of the or each glass molding plate. Due to the way it is made, float glass has a very high surface smoothness without the need for further surface treatment. The use of float glass with a nominal thickness between 4mm and 8mm, for example 6mm, is especially recommended because float glass of this thickness has the best surface quality.

In embodiments which use this preferred feature of the invention, it is preferred that multiple sandwiches made of a thermoplastic sheet and glass molding plates form a multi-layer sandwich, so that all of the thermoplastic sheets which constitute are conditioned simultaneously. This represents a more efficient use of the installation 30 and thus provides a more economical process.

Advantageously, the surface conditioning treatment is carried out before the bending of the thermoplastic material. This obviates the need to manufacture several curved mold plates each having the same predetermined curvature. The thermoplastic material can undergo flat surface conditioning, and this greatly simplifies the process.

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It has been found that the extent of the reduction in ripple or irregular stresses in the thermopastic material by controlling the cooling depends on the cooling rate during the first and second cooling periods, and also on the temperature at which begins uncontrolled cooling. For this reason, the adoption of one or more of the following preferred features of the invention is recommended: the duration of the or at least one, and preferably each, first cooling period is such that the temperature of the thermoplastic material falls from 20 ° C to 60 ° C; the duration of the 'or at least one, and preferably each, first cooling period is between 40 and 90 minutes; the sum of the durations of the or at least one, and preferably each, first and second cooling period is between 90 and 150 minutes; The duration of the or at least one, and preferably each, second cooling period is such that the temperature of the thermoplastic material drops from 100 ° C; the duration of the or at least one, and preferably each, second cooling period is such that the temperature of the thermoplastic material drops to 80 ° C.

For similar reasons, it is also advantageous to adopt one or both of the following preferred characteristics: during the or at least one, and preferably each, first cooling period, the thermoplastic is made or allowed to cool at a rate which does not exceed 2 ° C per minute, and which preferably does not exceed 1 ° C per minute; and during the or at least one, and preferably each, second cooling period, the thermoplastic material is made or allowed to cool at a speed which does not exceed 5 ° C. per minute, and which preferably does not Λ 10.

not exceed 2.5 ° C per minute.

It has been found that such cooling rates are entirely compatible with current industrial production yields.

It should be noted that the thermopastic material must not adhere to the molding plate either during the surface conditioning treatment or during the lamination treatment. If subsequent removal of the mold plate is in fact still practically possible, it would, however, almost inevitably damage the surface of the thermoplastic material if such adhesion occurs. Certain sheets of thermoplastic material do not normally adhere to the glass during the envisaged treatments, provided that a special coating or adhesive is used. Other materials can adhere if certain precautions are not taken to avoid this adhesion. In order to reliably prevent adhesion, it may therefore be preferable that the or each glass molding plate carries a non-stick layer, for example of a fluorocarbon product such as a fluoroalkyl- sulfonate, especially with C14 to C18 alkyl radical.

As already mentioned above, several problems concerned with certain embodiments of the present invention are all the more acute since the coefficient of linear thermal expansion of the thermoplastic material is higher. For this reason, embodiments of the invention in which the coefficient of linear thermal expansion of the material of the or at least one thermoplastic sheet is at least 43.10-6 ° C-1, i.e. 30 say about 5 times that of ordinary soda-lime glass, have a particularly important commercial value.

The commercial value of these preferred embodiments of the invention in which the material of the thermoplastic sheets has a relatively high coefficient of linear thermal expansion does not always depend solely on the resolution of the difficulties due to the importance.

“Tance of thermal contraction. Other properties of the thermoplastic material must also be taken into account.

In certain preferred embodiments of the invention, the at least one thermoplastic sheet is a polycarbonate sheet. Polycarbonates have, in addition to their coefficient of thermal expansion, particularly advantageous physical properties which justify their incorporation in laminated panels intended for different uses, in particular their ability to dissipate energy in the event of rupture or attempted rupture of the laminated panel. Metacrylates also have good properties for the purposes pursued, as well as certain polyvinyl chlorides and certain polyurethanes.

Other properties of the material of the thermoplastic sheets are also important. For example, high scratch resistance is of considerable commercial importance when handling a thermoplastic sheet, for example before incorporating it into a laminated panel, and especially for any exposed thermoplastic surface of a panel; finished laminated.

I Although it is sometimes possible to select the; material of the thermoplastic sheets for its good resistance; 25 abrasion resistance, because of its hardness, it is generally easier to increase the surface hardness of the sheet by applying an appropriate layer, for example by means of a synthetic resin which can, for one or | the other side, not suitable for forming the whole of the thickness of the sheet. For this reason, in preferred embodiments of the invention, the or at least one thermoplastic sheet carries at least one surface coating which is, or which can be treated to become, harder than the thermoplastic material of this sheet.

} In such preferred embodiments, it is particularly suitable that this treatment of the coating (s) 12.

surface (s) is carried out during surface conditioning. Such treatment can also be carried out during the bombing. This avoids the need for a separate treatment, and therefore achieves a saving of time and, in the case of a heat treatment, of heat energy.

A melamine surface coating is advantageous because of its excellent properties for the purposes that are being pursued. Other coating materials which can be advantageously used are polysiloxane resins and resins polymerizable by electron beam or by ultraviolet, for example those described in British patent application No. 2,131,324 A.

Advantageously, the surface portion of the bending mold and the edge of the bending mold are made up of separable elements. This arrangement facilitates the separation of the bending mold and the sheet of shaped thermoplastic material, and allows the use of different materials for the separable portions of the bending mold.

The materials constituting the portions of the bending mold are not critical. The shape of the surface portion of the bending mold having the desired dimensions and curvature is however very important. Preferably, said surface portion of the bending mold of desired size and curvature is formed by one face of a curved glass sheet. It is much easier to obtain, with glass than with any other material, a portion of the surface of the bending mold that is precise and of high surface quality. The border can be made of polytetrafluoroethylene or wood, for example.

The shape of the molding surface of the bending mold is particularly important in cases where the thermoplastic sheets must then be joined to a curved glass sheet. If the thermoplastic material must conform precisely to the glass to which it must be secured, it is obvious that this portion of! ; \! 13.

% bending mold must also comply precisely. The simplest way of ensuring this is that the said surface portion of the bending mold of desired dimensions and curvature is constituted by a face of the curved glass sheet to which the thermoplastic sheet must then be joined. Embodiments of the invention incorporating this feature are therefore preferred. The adoption of this preferred feature implies that the molding surface of the bending mold is changed for each thermoplastic sheet. This is not disadvantageous / even when manufacturing series, for example, of windshields which have to be adapted to automobiles of the same model, and which must therefore have the same nominal dimensions and curvature. In fact, in the production of a given series of curved panels for vehicle windshields, it is known that a small variation in curvature can be tolerated from one panel to another. However, it has been noted that precise conformity between a thermoplastic sheet and a glass sheet to which the thermoplastic sheet is to be joined is required and can often be more restrictive, and this therefore justifies the use as a form of bending of the curved glass sheet to which the thermoplastic sheet must be secured. Obviously, during the bending of a series of thermoplastic sheets with the same nominal curvature in such a production cycle, it is generally not necessary to change the border of the bending shape.

Preferably, the thermoplastic material is made or allowed to take the form of a concave molding surface. This has been found to simplify obtaining the desired surface conformance.

It is particularly important to obtain a | good compliance on the edges of the thermoplastic sheet! 35 after dimensioning. In order to promote this compliance, i! it is preferred that the curvature of the edge of the bending mold is greater than that of the edge of the surface portion | (i 14.

bending mold of desired dimensions and curvature. Surprisingly, it has been found that this arrangement gives the best results, even when the thermoplastic material is made or allowed to take the form of a concave molding surface.

The present invention includes a sheet of light transmitting thermoplastic material which has been bent by a method as described above; it includes a laminated light-transmitting panel comprising at least one sheet of curved glass secured to at least one such curved sheet of thermoplastic material, as well as a laminated light-transmitting panel manufactured by a method as described above.

The present invention extends to a device for giving a predetermined curvature to a sheet of thermoplastic material, characterized in that it comprises a bending mold having a surface of required dimensions and curvature and a border, means for heating and softening a thermoplastic sheet so that it conforms to said bending surface, while the edges of said sheet are partly supported by said edge, and means for allowing controlled cooling of the thermoplastic sheet. so that it retains the shape of the molding surface of the bending mold.

This device offers a very simple solution for conforming a sheet of thermoplastic material to a curved glass sheet. The edge of the bending form can be used to support a marginal portion 30 of a thermoplastic sheet so as to allow it to expand and contract freely during its heating and cooling, and in combination with the means allowing cooling. controlled, it allows more uniform cooling of the thermoplastic sheet, thereby reducing the risk of sheet distortion due to irregular contraction.

Advantageously, the molding surface of the mold of i 15.

% bending is delimited by one side of a curved glass sheet. It is very easy to make a curved glass bending shape with a high quality smooth surface. Preferably, said glass sheet is a sheet of float glass. The glass formed by the float process has a particularly good surface quality without it being necessary to subject it to special surface treatments.

The bending mold preferably has a concave molding surface. It has been found that it is simpler to conform a thermoplastic sheet to a concave face of given curvature rather than to a convex face of the same curvature.

In preferred embodiments of the invention, the curvature of the edge is greater than the curvature of the edge of the actual molding surface of the bending mold. It has been found that this arrangement is particularly advantageous when the curvature which it is desired to give to the thermoplastic sheet has a relatively small radius, especially in the vicinity of the edges of the molding surface of the bending mold, and when it is desired impart a left curvature to the sheet of thermoplastic material.

The present invention will now be described in more detail, by way of examples, with reference to the accompanying schematic drawings in which:

Figure 1 is a representation of an embodiment of a device according to the invention;

FIG. 2 is a graph showing temperature cycles of two specific methods for conforming a sheet of thermoplastic material to a curved glass sheet, according to the present invention; ί | Figure 3 is a schematic section of a t I device for conditioning the surface of a sheet of î! 35 thermoplastic material before conforming it to a glass sheet I;

FIG. 4 is a schematic sectional view of a device “i” “s 16.

tif to secure a curved laminated panel; and

FIG. 5 represents a pressure-temperature cycle which is used in a specific example of a method for conditioning the surface of a sheet of thermoplastic material.

FIGURE 1

In FIG. 1, an autoclave 1 contains an arch heating device 2 connected to a variable supply V. Inside the autoclave 1 there is a bending device which consists of two parts and is carried by several supports 3. The first part of the bending mold consists of a sheet, preferably made of glass, and preferably made of float glass, which delimits a molding surface 4 of required dimensions and curvature. A border 5 surrounds the edge of the glass sheet constituting the molding surface 4. A sheet of thermoplastic material 6 is shown, conforming to the molding surface 4 of the bending mold and to the surface of the border 5. The periphery of the molding surface 4 of the bending mold is substantially contiguous with the internal periphery of the edge 5 of the bending mold on the working surface of the molding form, and the edge 5 is arranged so that the radius of curvature of the edge 5 of the bending mold is smaller than that of the margin of the molding surface 4 of the bending mold.

EXAMPLE 1

It is desired to conform a polycarbonate sheet to the concave face of a glass sheet having a desired predetermined curvature. The glass sheet is placed in the device shown in FIG. 1 to constitute the molding surface 4 of the bending mold, and a planar, oversized sheet of polycarbonate is deposited above the molding surface 4 of the bending mold and the border 5. The heater 2 is then turned on, taking care to ensure that the glass sheet constituting the molding surface 4 and the polycarbonate sheet 6 are heated as uniformly as

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possible, and the polycarbonate sheet is heated to a temperature of about 155 ° C in about two and a quarter hours as indicated by the solid line in Figure 2. This temperature is maintained for about one hour, at the end of which the polycarbonate has collapsed and has taken the shape of the glass sheet constituting the molding surface 4; the power supplied to the heating device 2 is then reduced by acting on the variable supply V, so that the polycarbonate cools from its maximum temperature during a first period of approximately one and a quarter hours at a rate of 0.5 ° C. per minute. The feed is again reduced and the polycarbonate is allowed to cool at the rate of 1 ° C per minute for a second cooling period, also one and a quarter hours. When the polycarbonate has reached the temperature of 50 ° C., the autoclave is opened and the polycarbonate is allowed to cool naturally. The oversized edges of the polycarbonate are then cut to size. The polycarbonate sheet has a very high degree of compliance with the glass sheet 4, even on its edges.

As a variant of this example, it is desired to conform the polycarbonate to a convex face of a curved glass sheet. To perform this operation, the curved glass sheet is curved by a technique known per se in face-to-face contact with a second glass sheet, the concave face of which corresponds to the convex face of the first sheet. In this case, it is the second glass sheet which is placed in the device of FIG. 1 as a bending mold.

In a second variant of this example, the poly carbonate is subjected to faster heating and remains longer at its maximum temperature. This variant. method is particularly suitable for obtaining a left curvature and / or small radii of curvature, and it is shown in broken lines in FIG. 2. The temperature of the polycarbonate is brought to 140 ° C. in about an hour and then to 155 ° C in half an hour. It remains to

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this temperature for about two and a half hours, and is then subjected to the cooling cycle described above.

FIGURE 3 5 FIG. 3 represents an autoclave 7 in which is placed a sandwich comprising a sheet 8 of thermoplastic material, for example polycarbonate, placed between two flat molding plates 9 made of glass, made of 6mm float glass. Each glass molding plate 9 10 carries on each of its faces an optional non-stick coating consisting for example of a potassium fluoro-alkyl-sulfonate such as FC129 from 3M.

Such a non-stick coating can be deposited by spraying or by immersion.

It will be noted that several sandwiches made up of a thermoplastic sheet such as that indicated in 8 and of glass molding plates 9 carrying on each of their faces a non-stick coating can form a multilayer sandwich, so that all the thermoplastic sheets -20 ques which constitute it are conditioned simultaneously.

A torus made of elastomeric material is split to define lips 11 which are placed in contact with the margins of the molding plates 9 (external) so as to delimit a space 12 around the slices of the sandwich. This space 12 is in communication with a vacuum pump (not shown) via a pipe 13. The interior space 14 'of the autoclave 7 is connected to another pump (also not shown) via a second pipe 15. L The autoclave is provided with heating means which are not shown in the drawing.

EXAMPLE 2

An extruded sheet of polycarbonate 6mm nominal thickness is assembled between sheets of float glass 6mm thick serving as soft plates. The glass is previously coated on each of its faces with a non-stick agent. The sandwich is placed inside the device shown in Figure 3 and is j i | 19.

subjected to the temperature and pressure cycles shown in Figure 5.

The pressure P ^ 4 (solid line in FIG. 5), inside the autoclave, which acts on the main faces of the sandwich, is increased during a period of approximately one hour from 10 ^ Pa to 'at about 13.105pa, and it is kept at this value for about 3 hours.

It is important that the pressure P ^ 4 on the main faces of the sandwich is exerted by a fluid medium, such as gas, rather than by calendering or by the use of any form of mechanical press, because this ensures uniform pressure on these faces, which, in turn, leads to a more uniform treatment of the sheet of thermoplastic material.

The pressure Pj3 (broken line in FIG. 5) inside the duct 13 is reduced to around 1333Pa so as to avoid trapping air between the polycarbonate and the mold plates, and is maintained at this low value for approximately 2 hours, and it is then brought to approximately 13.10 ^ Pa, for example by opening a communication between the lines 13 and 15.

The temperature T (center line in Figure 5) of the polycarbonate sheet is raised to 140 ° C to 150 ° C over a period of 75 minutes, and is maintained at this value for 30 minutes. The polycarbonate is then allowed to cool at a rate of 0.5 ° C / min for a first period of approximately 60 minutes, and the cooling rate is then brought to about 1 ° C / min for a second cooling period of approximately 75 minutes. At the end of this second cooling period, when a total time of 4 hours has elapsed, the temperature; polycarbonate is about 50 ° C, and at this point, we | allows the pressure inside the autoclave to return to the | atmospheric pressure and we leave the sandwich, which is at! 35 indoors, cool naturally. The autoclave can then be opened after about another 15 minutes, and the polycar-! bonate and the molding plates can be removed.

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At the end of the process, the thickness of the polycarbonate sheet has been reduced to about 5mm and its surface is substantially flat, which makes it possible to see very clearly through it.

The resulting polycarbonate sheet is quite capable of satisfying the optical distortion test described in the "Belgian Monitor" of February 4, 1981, pages 1165 to 1170.

As a variant of this example, one face of a polycarbonate sheet 10 is pre-coated with a melamine resin. The coated polycarbonate sheet is then subjected to a surface conditioning treatment substantially identical to the cycle mentioned above; it is found after this treatment that the surface coating has polymerized in situ. The resulting surface coating is harder and more scratch resistant than uncoated polycarbonate. The same surface regularity is obtained.

The resultant surface-conditioned polycarbonate sheet is then curved to a predeter-20 mined curvature by a process such as that established in Example 1.

The product of the process of this example is perfectly suitable for being incorporated into a laminated glazing, for example as an external sheet of this glazing.

FIGURE 4 | 25 A device suitable for securing a panel [laminated by a method according to the invention is illustrated in I FIG. 4. In fact, this device is very similar to that illustrated in FIG. 3, and the same reference numbers are allocated to the different parts. of the device 30 in FIG. 4 than in FIG. 3.

> A sandwich consists of a curved sheet in | glass 4, a sheet of thermoplastic material which conforms thereto 6, and an intermediate film 16 of adhered material? thermoplastic sive. An is is shown in broken lines. Second adhesive film 17 between the sheet of thermal material? plastic 6 and a second curved glass sheet 18,: to secure, if necessary, this second sheet of i "i

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glass with laminated panel to form. If the sheet of thermoplastic material 6 is to constitute an external sheet, the finished laminated panel / the second film 17 of adhesive material is then omitted. The entire sandwich is secured by means of a torus 10 and placed in the autoclave as described with reference to FIG. 1. The first sheet of glass 4 can have a coating promoting adhesion and the second glass sheet 18 can carry an adhesion-promoting coating or an adhesion-preventing coating 10, depending on whether or not it must be secured in the laminated panel, as desired. Alternatively, the second glass sheet 18 is omitted.

It will be noted that it is possible, by a process according to this aspect of the invention, to join as many laminated sheets of thermoplastic material and glass between which films of adhesive material as desired, in a laminated panel. Only by way of example, the sandwich to be joined in a laminated panel comprises a sub-assembly 20 consisting of four glass sheets between which only films of adhesive material are inserted, this sub-assembly being provided with another adhesive film for fasten a single sheet of thermoplastic material to one side of this sub-assembly, j 25 EXAMPLE 3! In order to manufacture a curved vehicle windshield, a sub-assembly formed of a sheet of glass is formed; and a polycarbonate sheet between which is; inserted a polyurethane adhesive film. The glass sheet 30 mm is 6 mm thick and it can be pre-coated, if necessary, with a layer promoting adhesion. The polyurethane used is 2mm thick. The polycarbonate sheet used is the melamine coated product of Examples 2 and 1, as described above, the melamine coating obviously being opposite the glass.

] A glass molding plate 18 carrying a coating preventing adhesion is then assembled against the j .1

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polycarbonate sheet, and the assembly is enclosed in the split torus and placed in the autoclave, where it is subjected to pressure and temperature cycles in order to effect its joining into a laminated panel.

5 In a first period of the solidification treatment, the pressure acting on the main faces of the assembly is kept close to atmospheric pressure, while the spaces between the sheets are subjected to low pressure by reducing the pressure in the cana-10 lisation 7 to degas the sandwich. The temperature is increased in the autoclave. Then, in a second period, the interior 6 of the torus 4 is placed in communication with the rest of the interior 8 of the autoclave, and the pressure is increased there while the assembly is heated 15 to effect the final attachment .

It is found at the end of the process that the laminated panel is firmly secured and that its surfaces are substantially regular and allow to see clearly through it. The resulting laminated panel is entirely capable of satisfying the optical distortion test described in the "Belgian Monitor" of February 4, 1981, pages 1165 to 1170.

EXAMPLE 4

As a variant of Example 3, the sub-assembly 4, 16, 25 6 is simultaneously secured to the second film 17 of adhesive material (again 2 mm thick polyurethane) and to the second glass sheet 18 which, in this example is coated with an adhesion printing layer.

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Claims (35)

1. Method for imparting a predetermined curvature to a sheet of thermoplastic material in which such a thermoplastic sheet is curved by placing it against a face of a bending mold having a surface of the desired curvature, by heating the thermoplastic sheet and making it or letting it take the form of said surface, characterized in that such a thermoplastic sheet is oversized and is curved by placing it against a bending mold, a surface portion of which has the required dimensions and curvature and is surrounded by a border, the thermoplastic sheet is heated and made or allowed to take the form of said surface portion while its oversized margins are supported by said border, and then the thermoplastic sheet is cooled to a sufficiently slow pace to maintain such a shape, and in that the thermoplastic sheet is then dimensioned. 2. A method of manufacturing a light transmitting laminated panel comprising at least one curved sheet of thermoplastic material secured to at least one curved glass sheet, this method comprising bending said sheets to a predetermined curvature and their subsequent securing l to each other, characterized! in that such a thermoplastic sheet is curved by 2. a method according to claim 1, and in that the sheet; thermoplastic and at least one curved sheet of glass are assembled and the assembly is subjected to temperature and pressure conditions to effect the joining of the sheets into a laminated panel. 3. Method according to claim 2, characterized; in that the or each said thermoplastic sheet is dimensioned after joining in a laminated panel. 4. Method according to one of claims 2 or 3, characterized in that said laminated panel is of the type in which thermoplastic material is exposed on at least one main face of the panel. 4% 24. \ 5. Method according to claim 4, characterized in that such a glass sheet and such a thermoplastic sheet are arranged on either side of a thermoplastic adhesive film and a curved glass molding plate is brought into contact. with the or each exposed face of thermoplastic material and the assembly thus formed is subjected to temperature and pressure conditions such that the sheets of glass and of thermoplastic material join together. 6. Method according to claim 5 / characterized in that the or each curved glass molding plate and the or each curved glass sheet intended to form part of a laminated panel are assembled and curved at the same time at the desired curvature. 7. Method according to claim 1, characterized in that, while it remains in contact with said portion of the surface of the bending mold, the thermoplastic material is made or allowed to cool from its maximum »temperature during a first period at a first cooling rate, and then for a second period at a second cooling rate which is higher than the first. 8. Method according to one of claims 1 to 7, characterized in that the surfaces of a thermoplastic sheet are conditioned by a method comprising sandwiching the thermoplastic sheet between a pair of glass molding plates and the application to the sandwich of conditions of temperature and pressure so as to soften the thermoplastic material and to perform the required condition of the surfaces, and in that, before removing said molding plates, one makes or one allows the thermoplastic to cool from its maximum temperature for a first period at a first cooling rate, and then for a second period at a second cooling rate which is higher than the first. 9. Method according to claim 8, characterized V 25. "'A in that several sandwiches consisting of a thermoplastic sheet and glass molding plates form a multilayer sandwich sandwich so that all the thermoplastic sheets which constitute it are packaged 5 simultaneously. 10. Method according to one of claims 8 or 9, characterized in that the surface conditioning treatment is carried out before the bending of the thermoplastic material. 11. Method according to one of claims 7 to 10, characterized in that, during the or at least one, and preferably each, said first cooling period, the thermoplastic material is made or allowed to cool at a speed which does not not more than 2 ° C per minute, and which preferably does not exceed 1 ° C per minute. 12. Method according to one of claims 7 to 11, characterized in that, during the or at least one, and preferably each, said second cooling period, the thermoplastic material is made or allowed to cool at a speed which does not exceed 5 ° C per minute, and which preferably does not exceed 2.5 ° C per minute. 13. Method according to one of claims 7 to 12, characterized in that the duration of the or at least one, and preferably each, said first cooling period is such that the temperature of the thermoplastic material drops by 20 ° C at 60 ° C. 14. Method according to one of claims 7 to 13, characterized in that the duration of the or at least one, and preferably each, said first cooling period is between 40 and 90 minutes. 15. Method according to one of claims 7 to 14,; characterized in that the sum of the durations of the or at least! one, and preferably each, said first and said second cooling period is between 90 and 150: 35 minutes. 16. Method according to one of claims 7 to. 15, characterized in that the duration of the or at least one, and of "• * 26. preferably each, said second cooling period is such that the temperature of the thermoplastic material falls from 100 ° C. 17. The method of claim 16, characterized in that the duration of the or at least one, and preferably each, said second cooling period is such that the temperature of the thermoplastic material falls from 80 ° C. 18. Method according to one of claims 1 to 17, 10 characterized in that a sheet of float glass constitutes the molding surface of the or each glass molding plate. 19. Method according to one of claims 1 to 18, characterized in that the or at least one so-called thermoplastic sheet is a polycarbonate sheet. 20. Method according to one of claims 1 to 19, characterized in that the or at least one said thermoplastic sheet carries at least one surface coating which »is, or which can be treated to become, harder than the material thermoplastic of this sheet. 21. Method according to claims 8 and 20, characterized in that the treatment of the surface coating (s) is carried out during said conditioning of the surfaces. 22. Method according to one of claims 20 or 21, characterized in that said surface coating is a melamine coating. 23. Method according to one of claims 1 to 22, characterized in that said portion of the surface of the bending mold and said edge of the bending mold consist of separable elements. 23. V i 24. The method according to claim 23, characterized in that said portion of the surface of the bending mold of desired dimensions and curvature consists of one face of a curved glass sheet. 25. Method according to claims 2 and 24, characterized in that said portion of the mold surface of * 27. ♦ * ♦ bending of desired dimensions and curvature is constituted by one face of a curved glass sheet to which the thermopastic sheet is then secured. 26. Method according to one of claims 1 to 25, 5 characterized in that the thermoplastic material is made or allowed to take the form of a concave molding surface. 27. Method according to one of claims 1 to 26, characterized in that the curvature of the edge of the bending mold is greater than that of the edge of said portion 10 of the surface of the bending mold of desired dimensions and curvature. 28. Sheet of light transmitting thermoplastic material which has been bent by a method according to claim 1. 29. Laminated light-transmitting panel, characterized in that it comprises at least one curved sheet of glass secured to at least one curved sheet of thermoplastic material according to claim 28. 30. Laminated light transmitting panel 20 manufactured by a method according to claim 2. 31. Device for giving a predetermined curvature to a sheet of thermoplastic material, characterized in that it comprises a bending mold having a surface of required dimensions and curvature and a border, means for heating and softening a thermoplastic sheet | so that it conforms to said bending surface, while the edges of said sheet are partly supported by said edge, and means for allowing controlled cooling of the thermoplastic sheet so that 'It retains the shape of the molding surface I of the bending mold. 32. Device according to claim 31, character- ized in that the molding surface of the bending mold is delimited by one face of a curved glass sheet. 33. Device according to claim 32, character ized in that said glass sheet is a sheet of float glass. , 28. ”*. ♦ 34. Device according to one of claims 31 to 33, characterized in that said bending mold has a concave molding surface. 35. Device according to one of claims 31 to 5 34, characterized in that the curvature of the edge is greater than the curvature of the edge of said molding surface of the bending mold.